The present invention relates to an optical system of an optical pick-up employed in an optical disc device which is used for recording/reproducing data to/from a plurality of types of optical discs having different thicknesses of cover layers.
Various types of optical discs on which digital information is recorded at various densities and which have different thicknesses of cover layers have been widely used. For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
Since the CD (CD-R) and the DVD has the same disc size and thickness, it is desirable that a common optical pick-up is used for both of the CD (or the CD-R) and the DVD. However, in the optical system of optical pick-up, the amount of spherical aberration changes depending on the thickness of a cover layer of the optical disc being used. That is, the amount of spherical aberration changes depending on the type of the optical disc being used. Accordingly, to attain an optical system of an optical pick-up capable of supporting both of the CD (CD-R) and the DVD, it is required that the spherical aberration is corrected for both of the CD and the DVD.
A Japanese Provisional Publication No. 2000-81566 discloses an optical pick-up which is configured to correct spherical aberration caused by a difference in thickness between different types of optical discs. The optical pick-up in this publication includes an objective lens having a diffracting structure on its surface so that the change of the amount of the spherical aberration caused by the difference of the thickness of the cover layer can be corrected by wavelength dependence of action of the diffracting structure.
In general, since such an objective lens having the diffracting structure is difficult to fabricate, resin is used to form the objective lens. However, the resin has disadvantages in that its optical characteristic changes depending on temperature. That is, the objective lens formed of the resin has a problem that the optical characteristic deteriorates depending on the temperature variations.
A Japanese Provisional Publication No. HEI 11-337818 discloses another type of an objective lens of an optical pick-up. The objective lens disclosed in this publication is configured to correct a change of an optical characteristic due to temperature variations by a diffracting structure formed thereon as well as the change of the spherical aberration caused by the difference of the thickness of the cover layer of the optical disc. Since the objective lens in this publication has the above mentioned advantages, it has been used widespread at the present time.
However, the objective lens in the publication HEI 11-337818 has a problem to be solved that a working distance (i.e., a distance between the objective lens and a surface of the disc) becomes shorter when the CD or CD-R having the relatively thick cover layer is used in comparison with a case where the DVD having the relatively thin cover layer is used. It is understood that the optical disc device using this objective lens can not be downsized because a working distance for the DVD becomes longer when a required working distance for the CD (CD-R) is maintained.
Such a problem concerning the working distance is very important particularly in an optical disc device targeted for a portable device such as notebook computer because the portable device has a strict requirement for downsizing.
A Japanese Provisional Publication No. HEI 9-43510 discloses an optical system of an optical pick-up which is configured to solve the above mentioned problem concerning the working distance. The optical system disclosed in this publication is configured such that when the DVD is used, a collimated beam enters an objective lens to correct spherical aberration of the entire optical system, and that when the CD (CD-R) is used, a diverging beam enters the objective lens.
When the diverging beam is incident on the objective lens (when the CD is used), the spherical aberration caused by the objective lens changes to an undercorrected direction, while the spherical aberration caused by the cover layer of the CD changes to an overcorrected direction. In this case, the spherical aberration caused by the objective lens and the spherical aberration caused by the cover layer of the CD cancel each other. Thus, the spherical aberration is corrected both in the cases of the CD and the DVD.
When the CD is used, the working distance becomes longer than that in the case of the DVD because the diverging beam enters the objective lens when the CD is used. With this structure, the above mentioned problem that the working distance of the CD becomes too short can be solved.
Although, in the optical pick-up disclosed in the publication HEI 9-43510, the spherical aberration is corrected both in the cases of the CD and DVD, a coma caused by an off-axis ray becomes greater in comparison with a case in which a collimated beam enters the objective lens. The coma becomes greater particularly when the objective lens has an installation position error or when the objective lens shifts from a home position for tracking operation in a plane substantially perpendicular to an optical axis of the objective lens. In the followings, such a “lateral” shifting of the objective lens for the tracking operation and/or due to positioning errors is frequently expressed by the words “objective lens shift”.
If the optical pick-up in the publication HEI 9-43510 is used in a playback-only device, the optical pick-up delivers excellent performance because an numerical aperture (NA) on an image side (which is also expressed by the words “image side NA”) required for the reproducing operation is relatively low and thereby the coma caused by the objective lens can be kept within an allowable level. However, the optical pick-up may not be adequate for the recording operation because the recording operation requires a high NA on the image side and has a strict requirement for correction of aberrations.
A Japanese Provisional Publication No. 2000-338395 discloses an optical system for the recordlng/reproducing operation. The optical system uses a finite object distance when the CD is used. The optical system has a coupling lens configured to cause no aberration within its central area and to cause spherical aberration within its peripheral area. When an objective lens is not laterally shifted from a home position (i.e., when the objective lens shift does not occur), a beam passed through the coupling lens within its central area (i.e., a beam having no aberration) is incident on the objective lens. When the objective lens shift occurs, a beam passed through the peripheral area of the coupling lens is incident on the objective lens.
The spherical aberration given by the coupling lens when the objective lens shift occurs is used to cancel part of a coma caused by the objective lens and a cover layer of a disc.
However, according to the optical system disclosed in the publication No. 2000-338395, only one side of the coma which is distributed symmetrically about a point is canceled. Therefore, an adequate correction for the coma can not be attained. Further, a coma of a higher order remains in the optical system. Furthermore, the coupling lens disclosed in this publication is difficult to fabricate and to evaluate aberrations.